There are many human physiological characteristics which can be used to provide personnel identification for security purposes, such as fingerprint, retina, iris, DNA, or even face features. For all the devices that are capable of distinguishing some physiological characteristic of one person from others′, a fingerprint reader has the lowest cost and complexity, while the identification results are generally pretty good. In addition, the size of data required to store the minutiae of one fingerprint is small (ranging from 120 bytes to 2K bytes). This makes fingerprint identification devices widely accepted in many fields.
There are also many types of sensing techniques for capturing fingerprint. The popular ones are optical type and capacitive type. Optical fingerprint sensing modules utilize reflected light intensity from the surface of a finger to tell where the ridges and valleys are on the contact portion of the finger. The advantage of the optical technique is reliability and low cost. However, due to the size of the embedded optical lens, the form factor of a optical fingerprint sensing module cannot be kept small. It is difficult for the optical type sensor to be embedded in portable devices. The capacitive type fingerprint identification modules, on the other hand, are made out of silicon chips and can be made very compact. In some cases, when a fingerprint image can be fetched by slide scanning, the fingerprint sensor can be even thin and slim, too. The small form factor of capacitive type fingerprint identification module makes it suitable for portable applications such as access control badges, bank cards, cellular phones, tablet computers, USB dongles, etc.
Capacitive fingerprint sensor is based on the physics principle that the capacitance of a two parallel metal plates capacitor is inversely proportional to the distance between two plates. A capacitive fingerprint sensor consists of an array of sensing units. Each sensing unit contains a sensing plate. By using the sensing plate as one plate of the two-plated capacitor and a dermal tissue as another plate, ridges and valleys of a finger can be located by measuring the different capacitances. There are many prior arts related to the capacitive type fingerprint identification module. For example, the U.S. Pat. No. 6,114,862 discloses a distance sensor. It has a capacitive element in turn having a first capacitor plate which is positioned facing a second capacitor plate whose distance is to be measured. In the case of fingerprinting, the second capacitor plate is defined directly by the skin surface of the finger being printed. The sensor includes an inverting amplifier, between the input and output of which the capacitive element is connected to form a negative feedback branch. By supplying an electric charge step to the input of the inverting amplifier, a voltage step directly proportional to the distance being measured is obtained at the output. Although a structure of the sensor is simple, the amplifiers suffer uniformity problem and their energy efficiency is not good.
Another prior art is disclosed in U.S. Pat. No. 7,663,380. Please refer to FIG. 1A and FIG. 1B. A capacitive fingerprint sensor comprises a fingerprint capacitor CF, a reference capacitor CS, a first transistor 33, a second transistor 34, a third transistor 35 and a fourth transistor 36. The fingerprint capacitor CF has a capacitance that is either a valley capacitance CFV or a ridge capacitance CFR. The reference capacitor CS has a capacitance CS, and CFV<CS<CFR. The first transistor 33 is configured to pre-charge the reference capacitor CS. The second transistor 34 is configured to pre-charge the fingerprint capacitor CF. The third transistor 35 is configured to re-distribute the charges of the reference capacitor CS and fingerprint capacitor CF. The fourth transistor 36 is configured to output the voltage of the reference capacitor CS after redistribution.
FIG. 1A further tells the equivalent circuit of the fingerprint sensor in the pre-charge phase. In the pre-charge phase for the fingerprint sensor, the readout select line Cm (not shown) is asserted, the first transistor 33 and the second transistor 34 are enabled, and the voltages VA and VB pre-charge the reference capacitor CS and fingerprint capacitor CF, respectively. FIG. 1B shows the same circuit in the evaluation phase. In the evaluation phase for the fingerprint sensor, a readout select line Cm+1 is asserted, the third transistor 35 is enabled, and the electrical charges stored in the reference capacitor CS and fingerprint capacitor CF are redistributed. At this moment, a scan line is still asserted, the fourth transistor 36 is enabled, and the readout line outputs voltage depending on which portion of the human fingerprint, i.e., ridge or valley is detected. Apparently, the outputs voltage of the readout select line is larger if the ridge is detected, or smaller if the valley is detected. Thus, a fingerprint can be mapped based on the outputted voltages, varied with portions of the finger.
However, in practice, sensitivity of fingerprint sensing devices made by such capacitive fingerprint sensors is not high. When there is a protective layer on the top of the distance sensor, or the distance sensor is packaged in a molding compound, quality of fetched images gets worse.
Therefore, in order to resolve the problems mentioned above, an enhanced capacitive fingerprint sensing unit is desired.